The present invention relates generally to electrically resistive heating elements and more particularly to heating elements configured such that emissions of magnetic flux from the heating elements are reduced.
Most electronic devices experience changes in operating characteristics based on their operating temperature. For most applications, these variations are slight and can either be ignored or compensated for through calibration. However, there are instances in which environmental temperature regulation is required to ensure proper operation of an electronic device. For example, in many space applications where temperatures are extremely cold, environmental temperature regulation is required. At these extreme temperatures, electronic components may have operating characteristics that are quite different from their operating characteristics at room temperature causing them to malfunction or provide erroneous readings. Further, temperature regulation is also typically required for components that are particularly sensitive to variations in temperature. One example of such a device is a precision fiber optic gyro (FOG). Precision FOGs are particularly sensitive to changes in temperature. In precision applications, changes in the operating temperature of only a few millidegrees can affect the performance of the gyros significantly.
In many applications, strip heaters are used in temperature control systems for providing heat to electronic devices. Strip heaters include a resistive element that generates heat when a current is applied thereto. The heating element is either an elongated wire or trace of resistive material deposited on a substrate. The heating element is typically arranged in a pattern over a defined area to thereby provide uniform heat over the defined area. When current is applied to the heating element, heat is emitted from the strip heater.
While strip heaters are considered an inexpensive and efficient means of providing heat to electronic devices for environmental temperature control, there are some drawbacks to these devices. Specifically, as known in the art, when a current is applied to an elongated wire or trace, a magnetic field or flux is emitted from the heating element. This magnetic flux is problematic for several reasons. In terms of public safety, studies have linked high magnetic flux emissions as contributing to increased risk of cancer and other health problems. In addition, in terms of electronic device design, magnetic flux emissions, even at substantially lower levels, are known to negatively effect the performance of electronics and some types of fiber optics. Stray magnetic flux can also introduce output changes, drift, or noise into electronic components, which can corrupt data signals in an electronic device.
The magnetic flux emitted from a wire having infinite length is defined by the equation:
B=(xcexc0i)/(2xcfx80d) 
where:
xcex0=4xcfx80xc3x9710xe2x88x927 henries per meter;
i=amperes (AC and/or DC); and
d=distance from wire in meters.
As mentioned above, many conventional strip heaters employ elongated heating elements that are formed into patterns. These elongated heating elements can produce significant levels of magnetic flux. As seen from the equation above, the amount of flux emitted is inversely proportional to the distance d from the heating element. In most cases, the heating element is placed as close as possible to the item to be heated, thereby intensifying the amount of magnetic flux to which the element to be heated is subjected. Thus, although a strip heater element will serve to raise or regulate the operating temperature of the electronic device to a desired level, the magnetic flux emitted by the strip heater can adversely affect the electronic device""s operation.
Considerable effort, costs, and research is expended in electronic device design applications to shield devices from and eliminate sources of magnetic flux that may disrupt operation of the electronic device. For example, FIGS. 1A and 1B illustrate one conventional strip heater 10 having somewhat reduced magnetic flux emissions. As illustrated, the heater 10 includes a continuous heating element 12 that is folded in half to form two portions, 14 and 16. To reduce magnetic flux emissions, the two portions 14 and 16 are twisted about each other. When current is applied, as shown by the current arrows, current flows through the first portion 14 of the heating element 12 in one direction and through the second portion 16 in an opposite direction. In this configuration, because the current is the same magnitude through both portions, 14 and 16, magnetic flux emitted from the first portion 14 of the heating element is substantially cancelled by the magnetic flux emitted from the second portion 16.
Importantly, the amount of magnetic flux emission cancellation is related to the proximity of the first 14 and second 16 portions of the heating element to each other. In other words, the tighter the heating elements are twisted about each other, and the finer the elements are in terms of average diameter, the better the magnetic flux cancellation. Separations and air gaps between the first and second portions of the heating element, however, and the use of loosely wound, poorly-anchored, large diameter (0.010 inches or more) wires reduce the level of magnetic flux cancellation. As such, it is important to eliminate separations and air gaps between the first and second portions of the heating element for maximum magnetic flux cancellation.
Current heating element designs, however, do not properly address these problems. Specifically, for the most part, twisted heating element type strip heaters have been employed in heating blankets and similar applications. In these applications, heating elements are typically placed in the blanket material in a loose fashion. In this instance, the heating element is free to flex with the movements of the blanket. The flexing of a heating blanket also flexes the heating element allowing for separations and/or air gaps to form between the first 14 and second 16 portions of the heating element 12. For example, FIG. 1B illustrates a conventional heating element 12 in a flexed state. As can be seen, because the first and second portions are not fixed with respect to each other, air gaps 18 form in the flexed heating element, which thereby increase the amount of net magnetic flux emissions, (i.e., non-cancelled), by the heating element. While generally safe for human use, such heating blankets and similar devices can typically emit flux at levels tens or hundreds of times higher than many sensitive electronic devices can tolerate.
Some strip heating elements are substrate based, which means they are formed by depositing resistive traces on a substrate as opposed to wires. Advances have also been made to these substrate-based strip heaters to reduce magnetic flux emissions. In these systems, it is difficult to manufacture the heating element so that it has two portions twisted about each other. For this reason, conventional low flux substrate-based strip heater systems can be designed such that the resistance traces overlay each other on the opposed sides of the substrate. FIG. 2 illustrate a typical substrate-based strip heater system. Specifically, the strip heater system 20 includes a substrate 22 and first and second electrically resistive traces, 24 and 26. Importantly, the first and second traces are located on opposed sides of the substrate 22. The two traces each have opposed ends, (24a and 24b) and (26a and 26b), and bodies 24 and 26. Importantly, the bodies of each of the traces overlay each other in a corresponding pattern. Further, the opposed ends 24b and 26b are connected to each other by a via 28 extending through the substrate 22 to create a continuous heating element between the first ends 24a and 26a. In these systems, similar to the heating elements illustrated in FIG. 1A, current is applied to one end 24a of the trace and current (A.C. or D.C.) flows in opposite directions between the traces at any instant as shown by the arrows. This opposite flow causes a cancellation between the magnetic flux emitted by one trace and the magnetic flux emitted by the other trace.
Although these conventional substrate-based strip heating systems effectively reduce the amount of harmful magnetic flux emitted, present substrate-based strip heater designs do not maximize magnetic flux reduction. Specifically, as illustrated in FIG. 2, in many current substrate-based strip heater designs, the patterns of the first and second traces, 24 and 26, do not match each other at the location where they are connected to the via 28. Instead, there is a loop 30. This non-matching pattern area 30 results in a net magnetic flux emission between the portions, 32a and 32b, of the first and second traces, 24 and 26, because they do not sufficiently overly each other so as to maximize magnetic flux cancellation.
Additionally, for most substrate-based strip heater applications, it is advantageous to form connection pads, 34 and 36, on the same side of the substrate 22. To accomplish this, the contact pads 34 and 36, are offset from each other on the same surface, and the trace on the opposed surface, in this case the second trace 26, is redirected to the position of its corresponding connection pad 36. The trace 26 is connected to the connection pad 36 by a via 38 extending through the substrate. As illustrated, the pattern of the second trace 26 again diverges from the pattern of the first trace 24. Here again, because the patterns do not overly each other for the portion 32b, magnetic flux emissions are not effectively minimized.
In light of the problems with the conventional heating elements discussed above, the present invention provides both wire-based and substrate-based heating elements that effectively minimize the amount of magnetic flux emitted. Specifically, the heating elements of the present invention reduce the number of potential gaps, loops, and separations between the portions of a heating element that can cause a net magnetic flux emission.
For example, in one embodiment, the present invention provides a wire-based heating element having a body comprising first and second portions that are twisted about each other such that magnetic flux emitted from current flowing in one portion of the heating element is substantially cancelled by magnetic flux emitted from current flowing in the other portion of the heating element. Importantly, in this embodiment, the heating element further includes a bonding material that at least partially overlays the first and second portions of the heating element. The bonding material fixedly connects the two portions of the heating element that are twisted about each other together and restricts their movement relative to each other. This, in turn, reduces the potential for separations and/or air gaps between the first and second portions that would create a net magnetic flux emission by the heating element.
In addition to providing bonding material to fixedly connect the two portions of the heating element relative to each other, the present invention, in some embodiments, also provides a backing material for supporting the first and second portions of the heating element. More specifically, one embodiment of the present invention provides a backing material to which the first and second portions of the heating element are fixedly connected by the bonding material. In this embodiment, the backing material further reduces the amount of flex between the first and second portions of the heating element thereby minimizing the numbers of separations and/or air gaps that can be introduced between the first and second portions when the heating element is flexed. In a further embodiment, the backing material may further include an adhesive layer on a side opposite from the location of the heating element. The adhesive layer allows the heating element to be fixedly connected to a body to be heated by the heating element. Alternatively, the adhesive layer can be on the same side of the backing material as the heating element.
The present invention also provides embodiments in which the heating element itself is attached to the backing material in a serpentine pattern. The serpentine pattern effectively decreases the strain in the heating element due to tension forces that may be applied to the heating element during flexing. The pattern also provides for more uniform heating. In this embodiment, because the first and second portions of the heating element are fixedly bonded together and are further bonded to the backing material by the bonding material and because of the serpentine pattern of the heating element on the backing material, the present invention effectively minimizes the number of separations and/or air gaps that can be introduced between the first and second portions of the heating element that would create a net magnetic flux emission.
In some embodiments, the heating element of the present invention has a specific length corresponding to a desired heat output for the element. In these embodiments, the heating element may be required to fit within a selected area of the backing material to provide heat within that given area. In this embodiment of the present invention, the heating element, having a selected length L defines a serpentine pattern within the selected area having a width w and a length d. In this instance, the serpentine pattern creates zigzag portions that form triangles having a base 2xc3x97 and sides r. To properly fit the selected length L of the heating element in the selected area, the zigzag portions of the heating element have a selected triangle base width. Specifically, if the area of the backing in which the heating element is to be patterned has a width w and a length d and a heating element has an overall length of L, then half of the base length (x) of each triangle in the zigzag pattern will be defined by the following equation:
x=w[(L2/d2)xe2x88x921]xe2x88x921/2. 
In addition to providing wire based heating elements, the present invention also provides substrate-based heating elements. The substrate-based heating elements of the present invention reduce the number of loops and gaps between the first and second portions of the heating element, thereby minimizing the amount of magnetic flux emissions. For example, in one embodiment, the present invention provides a substrate-based strip heater having first and second heating element traces respectively located on first and second opposed sides of a substrate. The first and second traces have at least one of a corresponding pattern and a corresponding position relative to each other, such that magnetic flux emitted from current flowing in the first trace is substantially cancelled by magnetic flux emitted from current flowing in the second trace. The heating element of this embodiment further includes a first connection pad located on the first side of the substrate in electrical communication with the first trace. Additionally, the heating element further includes a second connection pad also located on the first side of the substrate in electrical communication with the second trace. There are first and second leads respectively connected to the first and second connection pads for applying electrical current thereto.
Importantly, in this embodiment, for the first and second connection pads to be located on the same side of the substrate, the second connection pad is offset from the location of the first connection pad by a selected offset distance. In this instance, a portion of the second trace is offset from the position of the first trace in order to connect to the second connection pad. To reduce the magnetic flux emissions from the offset portion of the second trace, the first lead extends along the first surface of the substrate overlying the portion of the second trace that is offset from the first trace. As such, the magnetic flux emitted from the offset portion of the second trace is effectively cancelled by the magnetic flux emitted from the portion of the first lead that overlies the second trace.
For example, in one embodiment, the second connection pad is located at position in front the first connection pad, and the second trace extends past the position of the first connection pad to connect the second connection pad. In this embodiment, the second trace substantially overlies the entire length of the first trace so that the magnetic flux from the first trace is substantially cancelled by the magnetic flux emitted by the second trace. Further, the first lead of this embodiment extends from the first connection pad over the portion of the second trace that extends past the first connection pad to the second connection pad to thereby substantially cancel the flux emitted from this section of the second trace.